Can not obtain session lock backup

Dear All,

Our Moodle server keep throwing error in the error_log file about session lock as below. 

Can not obtain session lock, referer: http://moodleserver/backup.backup.php

We don't set the Automated course backup in our Moodle. And, I am very sure that my Moodle load are low as we are in the term break. So, the session should be available. 

Any way to check why this error message appear. And, what trigger this message. 

We are using Moodle 2.3, running on Red Hat and Mysql server .

Thanks and Regards,

Yulstar